Power steering systems for reducing the steering effort required steer the vehicle are known, and one example of such systems is disclosed in Japanese patent publication (kokoku) No. 50-33584. According to this power steering system, the steering torque applied from a steering wheel is assisted by an output torque of an electric motor. By varying the gain for the detected steering torque applied to the steering wheel by the vehicle operator according to the vehicle speed and the road condition, the output torque of the assisting electric motor is adjusted so that an optimum steering assist torque may be obtained at all times.
When the vehicle encounters a strong crosswind or side wind, or runs into ruts or other irregular road surface features, the vehicle tends to deviate from the intended straight path of travel. When the vehicle travels over a low frictional coefficient road surface such as a frozen road surface or the vehicle is traveling at a low speed, the effort required to steer the vehicle significantly diminishes.
According to the conventional power steering system, however, the assisting torque is produced only when the vehicle operator has applied some steering torque to the steering wheel. Therefore, when the vehicle deviates from the intended straight path of travel due to crosswind, the electric motor will not produce any assisting torque by itself. Therefore, to prevent the deviation of the vehicle from the intended path, the vehicle operator must constantly apply a manual steering torque to the steering wheel. According to the conventional steering system, the assisting torque is generally small when the lateral acceleration and the yaw rate of the vehicle are large. Therefore, when the vehicle deviates from an intended path of travel, the steering input necessary to restore the vehicle to the intended path of travel becomes progressively greater as the lateral acceleration and the yaw rate of the vehicle increase.
It was previously proposed to compute a reference behavior of the vehicle in terms of a lateral acceleration and/or a yaw rate for a given steering angle and vehicle speed, detect the actual behavior of the vehicle, and apply a reactive steering torque so as to minimize the deviation of the detected vehicle behavior from the reference vehicle behavior. For details of such control arrangements, reference should be made to U.S. Pat. Nos. 5,528,497, 5,703,775, and 5,774,819 which are assigned to the common assignee, and the contents of these patents are hereby incorporated in this application by reference.
According to such a proposal, it is possible to control the vehicle in a stable fashion even when subjected to various interferences from outside. Also, the handling of the vehicle on a slippery road surface can be improved by preventing excessive steering. However, the sensors for detecting the behavior of the vehicle may produce signals which would cause a reactive steering torque to be produce even when such a reactive steering torque is in fact unnecessary. For instance, when the vehicle is being turned on a turntable or being transported on a lift or the like, this produces a lateral acceleration and/or a yaw rate which would be confused as an indication of vehicle behavior, and the resulting reactive steering torque is not only unnecessary but also causes discomfort to the vehicle operator.